I have noticed that my BB curls are more or less static even as I make progress in compound lifts. I'd say it's down to fatigue because when I do my one sessions of curls a week, I have completed 3-4 big lifts. The only direct tricep work I do are weighted dips, the bench and shoulder press give plenty of stimulation. Arm work is fine, but only after hitting legs, chest, shoulders and back.